Make entry_points behave the same across Python versions

[ocelotl]

The recently introduced entry_points function does not behave the same across Python versions and it is not possible to get all entry points in Python 3.8 and 3.9.

[lzchen]

@ocelotl

Do you have an example in which this does not work in 3.8 and 3.9 so we can try to reproduce?

[ocelotl]

@ocelotl

Do you have an example in which this does not work in 3.8 and 3.9 so we can try to reproduce?

Ok, so we have 2 issues:

1. We may need to go through all the entry points in some scenario, the current implementation of our entry_points function requires both group and name which a user who wants all entry points won't know, that's why all the implementations of the functions that give entry points in importlib or importlib_metadata have both group or name or their equivalents as optional arguments.
2. The replacements of pkg_resources functions that return entry points in importlib or importlib_metadata are very inconsistent across Python versions, for example 3.8 has an entry_points function that returns a dictionary, 3.10 returns a collection of entry point objects.

The idea behind this PR is to solve both issues by creating a private module that we can use in OTel components to access entry points that provides an uniform API and allows for selecting entry points by group, name or none of them.

[aabmass]

@ocelotl I have a high level question. Rather than add our own wrapper library, could we just use the importlib-metadata package instead regardless of version? Right now we have this conditional requirement

opentelemetry-python/opentelemetry-api/pyproject.toml

Line 30 in f05fa77

"importlib-metadata >= 5.0.0; python_version=='3.7'"

Once we drop Python 3.9 in the distant future, we could move to stdlib importlib.metadata. Are there any downsides to this?